Coupled systems of in vitro microfabricated organs-on-a-chip containing small populations of human cells are being developed to address the pharmacological and physiological gaps between monolayer cell cultures, animal models, and humans. These gaps present challenges not only in tissue and microfluidic engineering, but also in systems biology. For example, it must be determined how to model, test, and learn about the communication and control of biological systems at the scale of individual organs on chips. Allometric scaling provides some guidance, but appropriate biochemical and functional scaling of multiple organs and a universal cell-culture medium are also important to proper systems function and valid pharmacological interpretation.
Organ-on-a-chip technologies have advanced considerably in the past decade; however, understanding of biological scaling laws and how they apply to multiple, coupled organ devices has been largely ignored. To replicate human physiology and drug response with interconnected human organs-on-a-chip and larger human-like organ devices, each construct should have the correct relative size. Extensive literature describes differences in organ size between animal species whose body mass, M, spans 6 orders of magnitude. Organ size does not scale proportionally (isometrically) with M, but instead obeys a number of different allometric power laws that describe, for example, how as the animal's linear dimension L increases, its mass increases as L3, and hence the cross-sectional area of the bones must increase out of linear proportion. Metabolic rates scale as M3/4, blood circulation time scales as M1/4, and pulmonary and vascular networks exhibit M3/4 scaling (West et al., Science 276:122, 1997).
As organ devices are made smaller, scaling will ultimately fail, since individual cells have a fixed size, and immune cells, for example, function in isolation and at low densities. It is difficult to replicate the diameter of microcapillaries in tissue. The circulating volume of perfusate of an organ construct system must match organ size, lest metabolites, hormones, and paracrine signals be diluted to the point that each organ operates in a large reservoir independent of the other organs. Cellular heterogeneity, important to cellular signaling pathways in vivo, can be hard to maintain for long times in vitro. A universal media/blood surrogate is also needed to maintain multiple cell types, since most human cells are grown in media specific to the cell type and desired phenotype. Furthermore, devices should be mechanically and/or fluidly coupled and include sensing devices that can be used to evaluate the effects of compounds as they pass through each device.
The lung serves several physiological functions, and while its primary function is to enable optimal gas exchange, it is also involved in metabolic and immunological regulation. This functional complexity is reflected in its unique architecture that, to date has been difficult to simulate. A primary challenge in simulating the lung is the development of a scaffold that supports tissue growth while also simulating the structural characteristics of the lung. Although advances have been made in the art, the final goal of engineering and forming an in vitro lung organ mimic has not yet been realized.
Several limitations to traditional lung organ platforms include, but are not limited to, the inability to simulate the orientation or expansion of alveoli, the inability to develop a comprehensive lung organ platform (e.g., rather than just a small scale alveolar unit), the inability to incorporate pulmonary cells and microvascular cells into the device, the inability to temporarily support in vitro pulmonary gas exchange, the inability to restore pulmonary function for suitable periods of time (e.g., after implantation into an animal), uneven cell differentiation, poor vascular endothelial coverage efficiency, inefficient transport of dissolved oxygen and nutrients through the interior of organ tissue, and circulation leakage. Additionally, current approaches for determining drug toxicity in the art have only been tested on animal models, and data derived from human cell-based pulmonary organ is extremely limited.
Another challenge in the development of the lung organ is the differentiation of cells into the correct population dynamics to emulate lung diversity. Current techniques used in the art to achieve this goal produce low yields of pulmonary cells on three-dimensional (3D) synthetic scaffolds and also lack functional assembly of alveolar-like structures.